Human Memory


Whereas short-term memory lasts seconds, long-term memory endures for minutes, months or decades.  Aspects of long-term memory are then further categorized as ‘semantic memory’ (for facts) and ‘episodic memory’ (for events).  Forming an episodic memory involves three different stages encoding (the initial laying-down of its physical trace in the brain), storage (preserving the associations between connected brain cells) and retrieval (reconstruction of a remembered episode from its constituent parts).  Episodic memory typically occurs in conjunction with ‘autonoetic consciousness’: the sense people get of travelling back in time to re-live the moment.  Autobiographical memory-our personal experiences – is a specific form of long-term memory that involves both episodic memory for life events, plus semantic memory about how our lives have unfolded.


In neuroscience, the basic mechanism is called long-term potentiation (LTP).  This involves neurons ‘learning’ to fire electrical signals in sync, so that when one brain cell is firing, it also triggers the activation of a second, connected neurons.  Connections between neurons occur at the gap between the cells-the synapse-and require the synthesis of special enzymes, including the ‘memory molecule’.  Neurons in the hippocampus are particularly important in the laying-down and subsequent reactivation of connections needed for memory.  However, the pieces from which an individual memory is assembled are probably stored in separate areas of the cortex the occipital cortex (at the back of the brain) for visual information, and the auditory cortex (in the temporal lobes) for sound.


There are many reasons why an autobiographical memory might not come back into consciousness.  The basic reason for losing a memory is a failure of long-term potentiation.  In such cases, consolidation- stabilizing the memory by strengthening the neural connections-never occurs, so it never makes the transition from short to long-term memory.  Another important factor is whether the cues that were present at the moment of encoding, such as perceptual stimuli, match up with the cues present at the time of retrieval.  When a memory is laid down, perceptual stimuli are incorporated into the network of neural associations that end up being stored.  That’s why returning to a familiar place can trigger old memories associated with that location.  But if the right cues aren’t around at the moment of recall, the memory is effectively forgotten.

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